The present invention relates to a system for recovering drilling fluid from drill cuttings being produced by a drilling rig and recycling the recovered fluid to the rig drilling fluid storage and circulating system. More particularly the invention incorporates bed desorption, preferably fluid bed desorption, wherein forced gas convection heating is used to vaporize the drilling fluid. The vaporized fluid is then condensed and returned to the rig storage and circulating system.
Drilling for oil and gas produces drill cuttings. As drilling progresses, the cuttings are brought to surface by the circulation of drilling fluid through the wellbore. The cuttings are usually separated from most of the drilling fluid using vibrating screens, referred to as shale shakers, and centrifuges.
The cuttings retain a significant volume of drilling fluid on them after separation. It is desirable to clean the cuttings by removing the fluid and to return the latter to the rig""s drilling fluid storage and circulating system (herein referred to as the xe2x80x9crig fluid systemxe2x80x9d), for re-use. Otherwise the cuttings remain associated with drilling fluid and create a disposal problem involving environmental considerations. The drilling fluid often is oil based and may incorporate chemicals.
Current methods for disposing of cuttings contaminated with drilling fluid include: hauling the cuttings to a land fill and burying them; composting; bio-remediation; thermal desorption; and combustion.
Currently used thermal desorption processes for cleaning the drill cuttings are implemented after the rig has completed drilling. The processes commonly involve an eternally heated rotating drum for volatilizing the liquid. The off gases are burned rather than being recovered.
Another thermal method used involves fending the cuttings into kilns or fluidized bed reactors, wherein the fluid is directly burned off. Again, this is practised after drilling is finished.
These prior art thermal systems involve large scale, fixed capacity units that rely on constant rate, uniform composition feed. They are not adapted to clean cuttings while drilling is ongoing nor do they return separated drilling fluid to the drilling fluid system. Otherwise stated, they are not amenable to being integrated into the on-going drilling operation.
It is therefore the objective of the present invention to provide a thermal process for recovering drilling fluid from cuttings while drilling is on-going, so that the recovered fluid can be recycled to the rig drilling fluid system.
The present invention is directed to method and apparatus for cleaning drill cuttings as they are produced by a rig involved in drilling a wellbore. Drilling fluid coating the cuttings is to be recovered and recycled to the rig""s fluid system.
More particularly the coated drill cuttings are fed into a pressurized desorption chamber through which a stream of hot gas is pumped, to heat cuttings by convection and preferably fluidize them, thereby evaporating drilling fluid coating the cuttings. Optionally, indirect heat may be used to supplement the convective heat. Heating gas and drilling fluid vapours are removed from the chamber as an overhead stream. Part of the overhead stream is condensed to reclaim drilling fluid in liquid form. The reclaimed drilling fluid liquid is then recycled to the rig""s fluid system. Cuttings are removed from the desorption chamber as an underflow, preferably al variable rates to control the solids volume in the chamber below a maximum allowable level.
A non-condensed portion of the overhead stream vapours is re-heated and recycled for use as the forced convection heating gas. The recycled gases, containing condensible vapours such as hydrocarbons and water, maintain a low oxygen atmosphere in the desorption chamber, which reduces the risk of explosion.
Preferably the temperature in the desorption chamber is maintained in the range of 400xc2x0 F. to 600xc2x0 F., more preferably 540xc2x0 F. to 570xc2x0 F. As a result the temperature in the chamber is sufficient for evaporation but below the temperature at which coking, cracking and oxidation reactions become a problem.
Broadly stated, in one aspect the invention is concerned with a method for cleaning drill cuttings being produced by a drilling rig having a drilling fluid storage and circulating system, the cuttings being coated with drilling fluid, comprising: feeding the cuttings into a pressurized desorption chamber having an overhead vapor outlet and an underflow cuttings outlet; pumping hot pressurized healing gas through the chamber to contact and directly heat cuttings, by convection, so that drilling fluid on the cuttings is vaporized; discharging a gaseous mixture of heating gas and drilling fluid vapours through the overhead outlet; condensing part of the gaseous mixture to recover drilling fluid vapours in liquid form, thereby leaving part of the gaseous mixture as non-condensed gas containing condensible vapours; recycling the recovered liquid to the rig drilling fluid storage and circulating system; heating non-condensed gas and recycling it to the desorption chamber as heating gas; and removing cleaned cuttings from the desorption chamber through the underflow cuttings outlet.
In another aspect, the invention is concerned with a circuit for cleaning drill cuttings produced by a drilling rig having a drilling fluid storage and circulating system, the cuttings being coated with drilling fluid, comprising: a vessel forming a pressure desorption chamber having an overhead vapour outlet and underflow cuttings outlet; means for feeding drill cuttings into the chamber as they are produced by the rig; means for pumping hot pressurized heating gas through the chamber to heat cuttings by convection so that drilling fluid on the cuttings is vaporized to produce a mixture of heating gas and drilling fluid vapour, means for condensing part of the mixture leaving the chamber through the overhead vapour outlet, to produce liquid drilling fluid, thereby leaving part of the gaseous mixture as non-condensed gas containing condensible vapours, means for recycling liquid drilling fluid to the drilling fluid storage and circulating system; means for heating and recycling non-condensed gas to the chamber as heating gas; and means for removing cleaned cuttings from the desorption chamber.